1. Field of the Invention
The present invention relates to an electric power source apparatus using a fuel cell and a method of controlling the apparatus.
2. Related Art
As a result of recent development in electronic technologies, rapid popularization of mobile phones, notebook personal computers (hereinafter referred to as notebook PC), audio-visual apparatuses or mobile terminal devices is going on. These portable devices are driven by secondary batteries. New type of secondary batteries appeared; they were downsized and made high-energy density. The batteries have changed from seal-lead batteries, Ni/Cd batteries, Ni hydrogen batteries to Li ion batteries. In any batteries, development of active materials and high capacity battery structures have been made so as to increase energy density and to realize electric power sources with a longer service time.
In the portable electronic devices, an attempt for low electric power consumption has been made enthusiastically. However, it is necessary to add new functions to meet increasing user's demands. Therefore, there is a tendency that the electric power consumption of the portable electronic devices will increase further. Accordingly, electric power sources with a high energy density and a longer continuous service time are needed.
In order to realize an electric power source with a long continuous service time by secondary batteries, the charging time is a critical issue. Thus, demands for small sized generators that do not need charging are increasing; as a solution of this object, fuel cells have been considered.
Fuel cells that use as fuel hydrogen produced by reforming are widely known. These fuel cells are operated at 80° C. or higher, but fuel cells that use liquid fuel are Direct Methanol Fuel Cells (DMFC) that directly oxidize methanol at a fuel electrode. DMFC is safe with respect to temperatures; it is applied to mobile electronic devices as disclosed in patent document No. 1.    Patent document No. 1: Japanese patent laid-open 2002-32154
Although fuel cells have volume energy density (WH/L) and weight energy density (Wh/kg) superior to conventional secondary batteries in high density, output density of the fuel cell is low. Accordingly, if fuel cells are mounted on electronic devices that need high outputs, an area for electric generation must be designed for generating the high output. Fuel cells with such a large generation area would have a size and weight approximately equal or more to those of the conventional secondary batteries.
As one application for high output mobile devices there are external charger types. The external battery charger for mobile devices needs a capability capable of outputting electric power equivalent to an AC adapter for the mobile devices; the charger should meet performance of the total output for driving electric power for the mobile device and a secondary battery mounted on the mobile device. However, because of low output density of the fuel cell it is very difficult to realize downsizing and lightweight of the electric power source apparatus when the fuel cell is mounted on the external battery charger for the mobile device. On the other hand, although the fuel cells can be made small-sized and lightweight but have a small generation area and a small output power, the fuel cells used as an exterior battery charger that requires a guaranteed power of the AC adapter for conventional devices are difficult to employ; the system can be applied to devices that are designed not to require an electric power larger than a certain value, which lacks reality.